Various methods are used in radio communications systems for duplex operations. Conventional analog systems generally employ Frequency Division Duplex (FDD). Two different frequency bands are used; one for reception and one for transmission.
Time Division Duplex (TDD) is viable In digital systems. For example, the DECT system (Digital European Cordless Telecommunications) uses TDD. In this case the transmitter (Tx) and the receiver (Rx) operate on the same frequency band, only in different time slots.
In some future telecommunications systems such as the UMTS (Universal Mobile Telecommunication System) the system may have to be able to switch from the FDD state into the TDD state and vice versa. Two FDD bands would most likely be used in actual systems, the one upwards (uplink) to the base station and the other one downwards (downlink) from the base station to a mobile phone, for instance, and a third frequency band for TDD operations. Thus both the receiver and the transmitter must switch their operating frequencies when the radio part is coupled from the FDD state to the TDD state and vice versa. In the intermediate frequency a different bandwidth is typically used in the FDD state than in the TDD state. This means that the receiver must also switch the bandwidth of the intermediate frequency at the same time as the band of the operating frequency Is switched.
Where the receiver must switch its operating frequency band between two frequency bands, the changing over can be carried out by switching the frequency of the local oscillator. At the same time, the passband of the bandpass filter of radio frequency preceding the mixer should be switched so that it passes the right frequency band. Alternatively, two bandpass filters can be used, one of which is respectively coupled to the signal route using two switches. The switching of the frequency of the local oscillator can be considerable and it is not easy to implement using the same synthesizer. On the other hand, using two switchable synthesizers in the local oscillator is a far more complicated task.
The switch of the bandwidth of the intermediate frequency is normally carried out using two filters and two switches through which the required filter is coupled to the signal route. Alternatively, filters with different bandwidths can be coupled in series, as disclosed in U.S. Pat. No. 4,385,402.
An image reject mixer comprises a 3 dB radio frequency hybrid coupler with a 90 degree phase switch, an inphase power divider of the local oscillators, two mixers, and a 3 dB Intermediate frequency hybrid. The desired signal is summed inphasally in the intermediate frequency hybrid to another intermediate frequency output port and cancelled in the other Intermediate frequency output port. In the same way, the image signal is summed in the one port and cancelled in the port in which the desired signal is summed. Generally the port of the image signal is terminated and the port of the desired signal Is used as the Intermediate frequency output port.
FIG. 1 shows schematically a circuit arrangement according to prior art which can be used to switch the frequency of a receiver and which was described in a general way at the beginning. A radio frequency signal RF coming from an antenna (not shown), for instance, is directed to a bandpass filter (RF filter), and further into a mixer to be mixed by the frequency LO of the local oscillator, from where the intermediate frequency signal is directed to an intermediate frequency filter (IF filter), and from there to the output as a desired intermediate frequency output signal IF. The medium frequency of the intermediate frequency signal directed to the output can be switched by switching frequency LO of the local oscillator. However, also the RF and IF filters must then be switched so that they pass the desired band, respectively. Therefore, the RF and IF filters of FIG. 1 are duplicated. The frequency switching is carried out in the case of FIG. 1 by switches s1-s4 which are used to couple the desired RF filter and IF filter, respectively, to route RF-IF. The properties of the bandpass filters (RF, IF), i.e., the medium frequency and bandwidth are dimensioned in accordance with respective requirements in a manner known per se. As previously mentioned, the frequency switching of the local oscillator may lead to problems in practice.
In accordance with a first aspect of the present invention there is provided a circuit arrangement for switching the frequency range of a radio receiver, utilizing image frequency bands, whereby the frequency of a local oscillator Is adapted approximately to the middle of two selected frequency bands, characterised In that the circuit arrangement comprises
a mixer attenuating the image frequency, an intermediate frequency corresponding to the first frequency band being obtained from the first output of the mixer, and an intermediate frequency corresponding to the second frequency band being obtained from the second output, and
a switch for selecting a desired intermediate frequency corresponding to the frequency band either from the first or the second output of the mixer.
In accordance with a second aspect of the present Invention there is provided a circuit for switching the frequency range of a radio receiver between first and second frequency bands comprising:
means for providing a local oscillator or signal having a frequency between the first and second frequency bands;
means for mixing the local oscillator signal and a received signal to provide an output signal;
means for providing the output signal to a first output if the received signal is within the first frequency band and to a second output if the received signal is within the second frequency band; and
means for selecting the first and second output depending on the frequency band of the received signal.
The invention utilizes a circuit that could be used as an image rejection frequency converter to provide output signals down converted from a first frequency band to a first output and output signals down converted from a second frequency band to a second output, providing a simple arrangement for switching the frequency range of a radio receiver.
The Invention provides a circuit arrangement utilizing the image frequency bands for switching the frequency and bandwidth of the receiver, which can be used to simplify the circuit structure compared to prior art.
The circuit arrangement according to the Invention utilizes both outputs of a mixer which attenuates the image frequency, whereby one of the desired frequency bands is obtained from the xe2x80x9csignal portxe2x80x9d and the other desired frequency band from xe2x80x9cthe port of the image signalxe2x80x9d which is generally not utilized in prior art image rejection frequency converters.
The invention Is described in the following with the aid of the drawings in which: